The rising cost of energy in recent years has created a need for improving the insulation properties of both new and replacement roof installations. This need has resulted in an increased use of rigid insulation panels as an underlayment to an outer waterproof roofing membrane.
Depending on the design of the roof and the "R-Factor" required, the insulation may comprise sheets of: expanded or extruded polystyrene (EPS), vermiculite particle board, wood fiber board, fiber glass, urethane foam board, poly isocyanurate, or phenolic foam. The insulating panels must be firmly attached to the supporting substrate or structure of the roof so that the top surface of the panels presents a smooth, unbroken surface over which the roofing membrane may be applied and adhered.
Prior art fasteners used to attach the insulating panels have included nails having relatively large diameter heads, nail and washer assemblies, and screw and washer assemblies. Relatively large diameter washers are typically used with fasteners for this purpose to distribute the compressive force holding the panels in place over a greater area, thereby reducing the likelihood that the fastener will pull through the insulating panel.
When subjected to the forces resulting from vibration and movement of the connected pieces, thermal expansion/contraction cycles, and swelling and shrinkage due to changes in the moisture content of the pieces, fasteners can loosen and back out of the material into which they are driven. This is particularly a problem in roofing applications because a metal fastener provides an excellent thermally conducting path for heat flow between the outer surface and the underlying roof structure that greatly enhances thermal expansion an contraction cycle-induced back out.
If a nail or screw used to fasten the insulating panel in place should back out of the substrate, the head of the fastener is likely to at least abrade the overlying roofing membrane, and may eventually pierce the membrane, causing a leak in the roof. To reduce back out problems, roofing nails are usually provided with annular ridges around their shanks to increase their coefficient of friction relative to the material into which they are driven. Moreover, since a threaded fastener is less susceptible to backout, it is generally considered the fastener of choice for attaching insulation panels. However, even a screw is subject to loosening as heat thermally bridges the insulation by conduction from the head of the screw into the shank portion driven into the substrate, causing expansion and contraction of the fastener. Even if the fastener used to attach the insulation panel backs out only a fraction of its length, any rough edges or burrs on the head of the fastener may eventually abrade through the overlying membrane, degrading its waterproof integrity.
Use of prior art metal fasteners for attaching insulation panels also inherently reduces the effectiveness of the insulation. Du to the relatively high coefficient of thermal conductivity of metal compared to that of the insulation panels, each metal fastener of the prior art type tends to bridge the thermal barrier provided by the insulation, conducting heat between the interior of the building and the exterior surface of the roof. In view of the number (or density) of prior art insulation panel fasteners typically used per panel, the reduction in the insulating effectiveness in the panels caused by thermal bridging through the fasteners may be significant.
If a metal fastener extends beyond a substrate into a poorly ventilated space, thermal bridging wherein heat is conducted from the space to the outer surface of the roof can cause moisture to condense on the exposed end of the fastener because it is colder than the air in the space. Should the condensate drip onto an underlying surface, it can cause damage, e.g. water spots on interior ceiling tile. In addition, moisture collected on the fastener may also cause corrosion, perhaps resulting in eventual failure of the fastener.
The present invention is directed to reducing the above-described problems of conventional prior art fasteners used for attaching roofing insulation panels. The benefits and advantages that it provides compared to prior art fasteners will be apparent from the attached drawings and the description of the preferred embodiment which follows hereinbelow.